A conventional fiber-optic ring network comprises a plurality of nodes which are linked by optical fibers in a ring-like configuration. The ring may be designed so that data can be transmitted either clockwise or counter clockwise around the ring but not both.
Each node comprises a receiver and a transmitter. The transmitter in each node produces a modulated optical signal at a wavelength which can be detected by the receiver at the adjacent downstream node, where downstream is defined as the next node in the direction of data propagation around the ring. Data is transmitted from an originating node to a destination node by passing through each intermediate node on the ring between the originating node and the destination node. At each intermediate node, the data is detected by the receiver and regenerated in optical form by the transmitter.
In order for the entire ring to operate properly, the transmitter and receiver at each node must operate properly. Since there is only a single communications path around the ring, if any transmitter or receiver should malfunction, communication in the ring network would be severed.
One way in which a node can fail is for its transmitter to be stuck in the "off" mode. A fiber-optic ring network which overcomes this particular type of node failure is disclosed in A. Albanese "Fail Safe Nodes for Lightguide Digital Networks", The Bell System Technical Journal, Vol. 61, No. 2, February 1982, pp. 247-256. The fiber-optic ring of the aforementioned article is designed so that a small amount of light emitted by each node transmitter bypasses the immediately adjacent downstream node. Under normal circumstances, this "leakage light" is overwritten by the stronger light signal leaving the downstream node transmitter. However, if a node fails because its transmitter is stuck in the "off" mode, the light from the preceding node which bypasses the failed node can be properly detected by the receiver in the next succeeding node.
Thus, while the ring-architecture of the Albanese reference is useful for solving the problem of the stuck "off" transmitter, it is not capable of keeping a fiber-optic ring operating in the face of other types of node failures such as a transmitter which is stuck in the "on" mode. The stuck "on" mode of operation for a transmitter can take several forms including an output which comprises only digital "ones", or a valid looking output signal, which in reality contains garbage. Whatever its form, a stuck "on" transmitter is disastrous to the ring operation as a whole, since the communications path around the ring has been broken. The ring disclosed in the Albanese reference cannot solve this problem because the small amount of bypass light will always be written over by the stuck "on" transmitter.
Accordingly, an object of the present invention is to provide a fiber-optic ring network which is able to operate in the face of the failure of any one node, regardless of the particular cause of the node failure. More particularly, it is an object of the present invention to provide a fiber-optic ring network which is able to operate in the face of a stuck "on" transmitter failure as well as a stuck "off" transmitter failure.